1. Field of the Invention
The invention relates in general to an improved multi-touch sensor and electrostatic pen digitizing systems, circuits, and methods.
2. Description of the Related Art
Projected capacitive touch sensors typically include a substrate upon which electrodes for sensing a touch location are disposed. The substrate may be a durable glass having high optical transparency for viewing images displayed by an underlying display device that displays images such as graphical buttons and icons. When a user touches, for example with a finger or a stylus, on the outer surface of the substrate at a location corresponding to a desired selection displayed on the display device, the location is determined by sensing changes in capacitances to and between the electrodes.
In some projected capacitive touch sensors, the electrodes are arranged in rows of electrodes and columns of electrodes. The rows and columns are electrically isolated from one another via an insulating layer. A touch location is determined by driving electrodes of a first orientation (e.g., the column electrodes or drive electrodes) with a square wave signal (i.e., drive pulse). Sense circuitry coupled to the electrodes of the other orientation (e.g., the horizontal electrodes or sense electrodes) measures current flow between the electrodes due to mutual capacitive coupling that exists between the column electrodes and the row electrodes. The amount of current flow is directly proportional to the value of the mutual capacitance and therefore facilitates the determination of the mutual capacitance. The mutual capacitance between the intersection of a column electrode and a row electrode will change when a user touches the substrate in the vicinity of the intersection.
Typically, sense circuits for measuring the mutual capacitance operate by repetitively switching the sense electrodes to an input of an analog integrator circuit, which includes an amplifier with a feedback circuit that includes a capacitor that couples the amplifier output to the amplifier input. Such a circuit typically comprises a switch that couples the input of the integrator to the sense electrode just before each falling edge of the drive pulse that drives the drive electrodes and then uncouples just before each rising edge so as to integrate only signals of one polarity. The output of the integrator is then digitized and the digitized value is utilized to determine whether and where a touch has occurred.
However, the relative magnitudes of parasitic capacitances of the switch at the input of the integrator are large in comparison with the mutual capacitances between electrodes, which is typically measured in fractions of a pico-farad. To overcome the effects caused by the parasitic capacitances, a number of integration cycles are performed before a touch location may accurately be determined. For example, the integrator may integrate the signal measured on the sense electrode over two hundred or more cycles, which could take 1 ms or more for a drive pulse with a frequency of 200 kHz. The length of time to make a determination increases with the number of electrodes that must be measured, which may affect user experience for relatively large displays that typically have a large number of electrodes to measure, relative to smaller pCap displays used in mobile devices.
A touch location can also be determined by driving electrodes of a first orientation only (e.g., the column electrodes) and sensing the current change only to the driven electrode. The sense circuitry measures current flow changes to the electrodes due to electrodes self-capacitive coupling that exists between the driven electrode and impedance paths to ground which can include paths from the electrode to other electrodes. The amount of current flow is directly proportional to the value of the impedance paths and therefore facilitates the determination of the self-capacitance. The self-capacitance will change when a user touches the substrate in the vicinity of the electrode altering the impedance paths to other electrodes but also adding new paths through the user to any ground potential.
In typical multi-touch systems the self-capacitive signal-to-noise ratio is much larger than the mutual capacitances due to the fact that that the self-capacitive signal contains the drive signal, the sensor parasitic capacitances, as well as the touch signal energy change whereas the mutual capacitance signal is much smaller as it only contains the cross parasitic capacitance and touch signal energy change. Also in typical systems the self-parasitic capacitances is large because the surrounding channels are effectively grounded as only one signal is driven at a time. Surrounding channels in this case are the channels adjacent and also crossing channels on a two axis system including the delivery traces to the touch area which are typically a very large portion of this parasitic capacitance. These parasitic capacitances interact with the pulse or square wave driving and sampling which contain high frequency harmonics. These harmonics contain a significant portion of the touch energy change which attenuates faster than the fundamental when passing down a RC impedance chain and back causing considerable signal and increasing signal loss as the touchscreen impedance rise.
In some previous capacitance touch sensor systems, the self-capacitance measurement has been used with guard electrodes where the adjacent electrodes are driven with the same signals so as to shield the electrode of interest from the current flow of the impedance paths of the target electrode and adjacent electrodes. The shielding also blocks current flow to further adjacent paths as the voltages of the adjacent shield electrodes supply almost all of the current and charging of these further capacitances and impedance paths. When correctly executed this self-capacitance measurement and shielding can be used to reduce the error signal due to contamination by a conductor such as salt water, which will tend to add to the users touch and will tend to bridge energy to surrounding impedance paths. This adjacent shield method does not block the alternate axis channels near and crossing the driven trace and so only shields about half the possible impedance paths. The ability to measure a touch in the presence of salt water contamination on the touch sensor is in some cases such as industrial, marine, or military applications, highly desired but does not work well on the current solutions available.
Typical touch control circuits have the ability to measure the different modes self or mutual capacitance or even to measure only the un-driven state of the electrodes as a method of receiving only external signals. But aside from driving a few shield electrodes or a drive/sense pair typically the modes of sampling can occur only one mode at a time. The length of time to make a determination for each mode increases with the number of electrodes that must be measured, which may affect user experience for relatively large displays that typically have a large number of electrodes to measure, relative to smaller pCap displays used in mobile devices.
Sigma-Delta Analog to Digital Converters (ΣΔADC) have been known for some time but have recently become very popular as programmable logic clock speeds have improved to the point where very good conversion function is possible. Many new ideas and work centered on improving these converters speed and functionality has been in an effort to allow this more digital conversion method to replace the more standard analog techniques. In the touch realm many improvement patents have been granted around incorporation of known capacitive sampling techniques and Delta Sigma conversion of analog to digital.
U.S. Pat. No. 8,089,289 has an example of prior art technology using a Delta Sigma Converter and showing mutual capacitive scheme using square wave drive and switched capacitor function with rectification in two embodiment drawings of the same function, as shown in FIG. 20.
U.S. Pat. No. 7,528,755 shows an example of prior art technology using a Delta Sigma Converter and showing scheme capable of signal drive or measure technique selectable via a mux as shown in FIG. 21.
U.S. Pat. No. 8,547,114 shows an example of prior art technology using a Delta Sigma Converter and switched capacitor techniques as shown in FIG. 22.
U.S. Pat. No. 8,462,136 shows an example of a prior art strategy, this state of the art mutual capacitance multi-touch system with simultaneous digital square wave patterned transmission and simultaneous receive with synchronous demodulation and pen capable, as shown in FIG. 23. This system does not allow multi-mode concurrent touchscreen sampling, does not have true simultaneous sampling due to each row using a different bit pattern which effectively scrambles the noise distribution on receipt, is not capable of self-capacitance measurements, and due to the use of square wave drive has a receive signal spectrum that contains the primary frequency as well as its harmonics which necessitate lower trace impedance to prevent attenuation of the higher harmonics across the panel.
The systems shown do not allow multi-mode concurrent touchscreen sampling, do not have true simultaneous sampling, use pulse or square wave sampling which does not allows for anti-alias filtering and have high frequency components necessitating lower touchscreen impedances, and all but the last uses mux arrays with high parasitic capacitances.
Therefore, a need exists for a much faster sampling method that can acquire data simultaneously for different modes of, for example, self, mutual, and pen, and with simultaneous sampling of the different channels.
Also, in some applications, to reduce the sample time via signal to noise ratio improvement where possible, continuous sampling schemes and advanced filter methods, modulation and demodulation schemes, and digital domain methods are needed. To keep the cost and power usage as low as possible the circuitry should be as much in the digital realm as possible.
Finally, many different touch sensors are now available that work through the measurement of changes to impedance, and providing a system that can handle multiple sensor types and configurations, including those currently known and those to be developed in the future, is also greatly desired.